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Scott, Epoxy fares fairly well in sunlight, but eventually gets chalky. For the amount of time they are in the sun, it does not make much difference. If you can topcoat the epoxy with polyurethane, you will have the durability, gloss, and UV resistance. However, as you must know, even epoxy can get scratched through. Hey, thanks for the info on O2 whips. ---Steve At 06:42 AM 8/11/00 -0700, you wrote: >Thanks for straightening that out. > >I have seen several sets of tanks over the years that were powder coated, >and they *appeared* to be very durable. I was unaware of the migration >issue. I was aware of the heat issue. > >I have used epoxy paint on AL tanks with good results, however. > >What is the problem with the UV and epoxy? Its OK to use it as a primer, but >not as a top coat? > >Thanks! > >Scott > > > >----- Original Message ----- >From: <Gilldiver@ao*.co*> >To: <techdiver@aquanaut.com> >Sent: Thursday, August 10, 2000 2:25 PM >Subject: Re: ScubaPro Faber Tanks-Paint Bubbles and Rust > > > > After reading some of the posts on tank corrosion I thought it was time >for > > a little science on coating and corrosion. > > > > This is probably more then you ever wanted to know about paints. > > > > First, Powder Coating: Powder coating is the process of applying a powder > > paint like material to an electrically conductive surface. The powder is > > then heated so that it can melt and polymerize into a uniform coating that > > can be very corrosion resistant because of barrier protection of the > > underlying metal. That means that water cannot get to the metal. However, > > if a scratch breaks through the coating and there is no sacrificial >element > > under it (like zinc galvanizing) corrosion will start immediately and can > > travel under the coating. > > > > For tanks there are two problems with powder coating, the first is the > > requirement for electric conductivity. Powder coats are applied by a >static > > electric charge and many primers are not conductive. Therefor, primers are > > generally not used. This results in the application of powder directly >onto > > the tank surface without any primer. On bare steal this is not a problem >but > > on zinc you will have a hard time getting really good adhesion due to the > > oxide layer that forms on the galvanized surface almost instantaneously >when > > it is exposed to air. If there is any remaining paint for the old coat you > > will have spotting of the new coating. > > > > The second problem is from heating to melt the powder and cure it. On >steel > > tanks there is no problem on aluminum their is a BIG problem. Aluminum > > alloys are just that ALLOYS. They contain other metals like iron, copper, > > zinc, magnesium, and manganese. Whenever these alloys are exposed above >275F > > you will start to get eutectic melting. This is melting of these alloying > > metals within the crystal structure of the aluminum. Metals like copper >and > > zinc will move along the crystal structure and pool on a microscopic >level. > > In stressed parts like tanks this pooling forms weak spots were cracks can > > start easily. Above 250F you will also start to change the crystal >structure > > of the aluminum which in itself can form stresses and cracking. As powder > > coating typically is done above 300F, powder coating should never be done >on > > an aluminum tank. If an aluminum tank is exposed to temperatures above >250F > > it should be scrapped. > > > > So powder coating will work somewhat on steel tanks but never on aluminum. > > So what can you do? > > > > Try looking around the local airport for a small scale aircraft paint > > facility that will do small jobs. Have them completely strip the tanks >with > > a chemical stripper or Plastic Media Blast (PMB) system. Never use sand > > blasting on aluminum or galvanized tanks. Have them apply an aircraft >grade > > chromated primer and then either you or them apply a high quality aircraft > > urethane topcoat. Never use an epoxy topcoat as it will react with the >suns > > UV rays and start breaking down. The chromated chemicals in the primer >will > > act as a sacrificial element protecting the underlying metal and if a >scratch > > breaks through it will protect the bare metal in the scratch for a long >time. > > These primers will protect aluminum and steel in salt spray chambers for >at > > least 2000 hours. That would be about 5 years on a boat. > > > > The primers to look for are compliant with US Federal specification > > TT-P-1757 (Zinc Chromate) or Mil-P-23377 solvent base epoxy or Mil-P-85582 > > water base epoxy. Both of the Mil primers contain either Strontium >Chromate > > or Barium Chromate. For top coat I recommend a material meeting >Mil-P-85285. > > This system of chromated primer and high quality urethane top coat will >give > > very good protection and is quite resistant to impact damage (think about >a > > jet fighter at mach 2 in a rain storm). > > > > > > So get a good clean surface, prime it, and topcoat it. > > > > Pete Johnson > > -- > > Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > > Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. > >-- >Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. >Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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